1. Field of the Invention
The present invention relates to a novel type of semiconductor component. The present invention more specifically applies to power components and to protection components intended to handle high voltages, such components being generally called discrete components, although several such components may be provided on the same chip, and/or they may be associated with logic circuits provided on the same chip.
2. Discussion of the Related Art
FIGS. 1A and 1B show as an example a perspective view and a cross-section view of a conventional vertical power diode. This diode is formed from a substrate comprising a heavily-doped N-type region 1 (N+) and a lightly-doped N-type region 2 coated with a P-type layer 3. The upper surface is coated with an anode metallization 4 and the lower surface is coated with a cathode metallization 5. Reference numeral 6 designates an insulating layer.
FIG. 2 is a perspective view of a vertical power thyristor. The thyristor comprises a lightly-doped N-type substrate 10. On the upper surface side is formed a P-type well 11 containing an N-type cathode region 12. On the lower surface side is formed a P-type anode layer 13. An anode metallization MA, a cathode metallization MK, and a gate metallization MG are also provided. To prevent for the anode metallization from short-circuiting to substrate 10, or to separate this thyristor from a neighboring component, a peripheral P-type insulating wall 15 is generally provided.
It should incidentally be noted that in the present description, the term “diode” designates a PN or Schottky diode intended to be used as a power, protection, or avalanche diode. A diode is a bipolar component having two terminals intended to be connected to elements of a dielectric or electronic circuit, discrete or integrated, that, according to cases, conducts a forward current and blocks a reverse current (rectifying diode), or conversely, conducts a reverse current when the voltage thereacross exceeds a given threshold (protection diode). In the thyristor of FIG. 2, the separation surface between P-type insulating wall 15 and N-type substrate 1 is never intended to be conductive, but only either to enable the component periphery to be entirely at the rear surface potential, or to insulate well 1 from an adjacent well containing another component. This separation surface is not associated with terminals intended to be connected to elements of an electric or electronic circuit. Such a separation surface does not form a diode (sometimes conductive, sometimes blocked) connected to terminals of connection to a circuit.
A disadvantage of vertical components is their on-state resistance. Indeed, the thicknesses of the various layers and regions are optimized according to the desired diode characteristics. In particular, the thickness of N-type layer 2 (diode) or 10 (thyristor) must be sufficiently high for the component to have a desired breakdown voltage but must also be as small as possible to limit the on-state resistance of the component. In the case of a diode, N+ layer 1 has no active function in the diode operation. It is only used to ensure an ohmic contact with the metallization and is used to reduce the diode's on-state resistance linked to the fact that a silicon wafer has, in current technologies, a thickness of from 300 to 500 μm, in most cases much greater than the desired thickness of N layer 2 (for example, 60 μm to hold 600 V). In the case of the thyristor, the thickness of layer 10 is also required by the thickness of the silicon wafer and various means, often complex, are used to reduce it.
Another disadvantage of vertical components is that the surface area of the active junctions is linked to the semiconductor chip surface area taken up by the components, the junctions being horizontal (in planes parallel to the main diode surfaces).
Further, such components intended to handle high voltages pose many problems to ensure a proper breakdown voltage at the periphery of the semiconductor or Schottky junction, as well as to insulate the entire component and ensure its protection (insulating wall).
A PNN+ diode and a thyristor have been described as an example only of vertical components, the problems indicated hereabove generally relating to vertical power or high-voltage components, for example, Schottky diodes, bi-directional components, or MOS-type voltage-controlled components.